Chlorine dioxide is widely used as a disinfectant in water treatment/purification and as a bleaching agent in pulp and paper production. There are a variety of chlorine dioxide generator systems and processes available in the marketplace. Most of the large scale generators produce chlorine dioxide by reacting a chlorate ion with hydrogen ions and an electron. This reaction can be represented by: EQU ClO.sub.3.sup.- +2H.sup.+ +e-.fwdarw.ClO.sub.2 +H.sub.2 O
These generators use sodium chlorate and/or chloric acid to provide the chlorate ions. Hydrogen ions are supplied by sulphuric or hydrochloric acid. The electron can be provided by a reducing agent such as methanol, sulphur dioxide or sodium chloride.
Unfortunately a problem exists in that these generators produce undesirable by-product salts such as sodium sulfate ("salt cake"), sodium bisulfate and/or sodium chloride. U.S. Pat. No. 4,081,520 discloses a process of producing chlorine dioxide in which sulfuric acid is reacted with sodium chlorate and methanol. Sodium ions from dissociated sodium chlorate react with sulfate ions from sulfuric acid to form by-product sodium sulfate. For every mole of sodium chlorate that reacts with a mole of sulphuric acid, one mole of sodium sulfate is produced. As the concentration of sodium sulfate rises it begins to precipitate out of solution and must be removed. The removal of sodium sulfate or other commonly formed salts represents an additional expense in the production of chlorine dioxide. Increasing the production of chlorine dioxide generally means increased capital investment to process the additional amounts of by-product salt produced.
U.S. Pat. No. 3,563,702 describes a process for Producing chlorine dioxide where an alkali metal chlorate, an alkali metal chloride and a strong acid are reacted in the presence of a catalyst. This reaction produces an effluent containing sodium sulfate which must be removed from the reaction.
U.S. Pat. No. 4,486,399 to Lobley recognizes the Problem of by-product salt formation in chlorine dioxide generators. Lobley discloses a method of producing chlorine dioxide that uses minimal amounts of sulfuric acid and feeds sodium chlorate to the generator as a solid. In this manner sulfate salt content in effluent from the generator can be reduced.
Another process of producing chlorine dioxide is disclosed by Schlumberger in U.S. Pat. No. 3,810,969 which teaches a method of making chlorine dioxide from a solution of chloric acid. Chloric acid is made by passing alkali metal chlorate through a cation exchange resin. The resulting chloric acid is reduced using hydrochloric acid in the presence of a catalyst. Sodium bisulfate and sodium sulfate are produced as by products requiring disposal in this reaction.
A process reducing the amount of by-product salt is therefore commercially desirable. In an effort to reduce the expense of removing and disposing of undesirable salts in the effluent of chlorine dioxide generators, new processes have been developed which reduce the quantity of by-product salts. U.S. Pat. No. 4,798,715 to Hardee et al discloses a process of producing chlorine dioxide in an electrochemical cell which reduces the salt content of effluent from the chlorine dioxide generator.
Twardowski, U.S. Pat. No. 4,806,215 discloses a chlorine dioxide generating system in which effluent containing sodium chloride from the chlorine dioxide generator is recycled though an electrochemical cell to produce hydrochloric acid and sodium hydroxide. The hydrochloric acid can then be reused in the generator to make chlorine dioxide.
Lipstajn et al discloses a means of making chlorine dioxide whereby sulfate salts are not formed. In WO 92/03374 Lipstajn teaches the reduction of chloric acid in the substantial absence of sulfate ions to make chlorine dioxide.
While methods are known to make chlorine dioxide with reduced levels of salts in the resulting effluent, such processes are often limited by the efficiency of the electrochemical cells used in such processes. In these processes the source of chlorate ions is often sodium chlorate. In the electrolytic formation of chloric acid from sodium chlorate, the electrochemical current efficiency is determined by the ratio of sodium to hydrogen ions entering and leaving the first ion exchange compartment in a three compartment cell, or the anode compartment in the case of a two compartment cell. Hydrogen ion transport into the cathode compartment is a cause of the current inefficiency in electrochemical processes using cells in the Prior art.
Thus two problems exist. First the production of by-product sulfate salts in effluent from a generation reaction and second, the efficiency of the electrochemical acidification reaction. As a result of these problems there exists a need for a process of producing chlorine dioxide which does not produce undesirable salts and which operates at a high efficiency.